1. FIELD OF THE INVENTION
This invention relates to a limiter circuit adapted for use in a radio receiver mounted on a car or in a CB transceiver, which can suppress pulsating noise superposed on a demodulated signal.
2. DESCRIPTION OF THE PRIOR ART
A radio receiver mounted on a car or a CB transceiver is liable to pick up pulsating noise generated by the internal combustion engine.
In general, therefore, such a car-mounted device has a limiter circuit for suppressing pulsating noise provided in its receiving circuit.
Such a limiter circuit as currently used in the fields of the art, is shown in FIG. 1. Namely, an inverse parallel circuit of a series connection of a limiting diode 161 and an associated bias source 17 for determining the conduction starting voltage of the diode 161, i.e. one limiting level, and a series connection of a limiting diode 162 and an associated bias source 18 for determining the conduction starting voltage of the diode 162, i.e. the other limiting level, is connected between the earth and a demodulated signal transmission line 15 for transmitting a demodulated signal (or a modulated signal) e.sub.i received at an input terminal 12 to an output terminal 14 through a signal source resistor 13. In case of reception, the diodes 161 and 162 are inversely biased to their cut-off states under the control of the bias sources 17 and 18 so that if the pulsating noise superposed on the demodulated signal e.sub.i exceeds the level of, for example, 100% AM modulation, the noise may turn the diodes 162 and 161 conductive. Accordingly, the pulsating noise exceeding the level of 100% AM modulation is short-circuited to the earth through the diodes 161 and 162.
FIGS. 2A and 2B show waveforms useful in explaining the limiter operation described above. In the figures, reference numeral 1 indicates a dc voltage V.sub.A corresponding to the reference level in the absence of modulation, derived at the junction point A of the demodulated signal transmission line 15 and the limiting diode 161 or 162; 2 a limiting level defined by the sum (V.sub.17 +V.sub.Z) of the bias voltage V.sub.17 of the bias source 17 and the threshold voltage V.sub.Z of the limiting diode 161; 3 a limiting level defined by the difference (V.sub.18 -V.sub.Z) between the bias voltage V.sub.18 of the bias source 18 and the threshold voltage V.sub.Z of the limiting diode 162; 4 a waveform of a demodulated signal with its AM carrier of 100% modulation; 5 and 6 noise pulses superposed on the demodulated signal waveform 4; 4' a waveform of a demodulated signal with its AM carrier of less than 100% modulation; and 5' and 6' noise pulses superposed on the demodulated signal waveform 4'.
However, the limiting effect of such a limiter circuit with constant limiting levels as described above varies largely depending on the positions in which the noise pulses are superposed on the demodulated signal, that is, whether the positions are at the peaks of the demodulated signal or at the throughs of the signal, or depending on the degree of modulation of a signal to be demodulated. Thus, the limiter circuit cannot provide its highest possible performance. Namely, assuming that the limiting levels are so set as indicated at straight lines 2 and 3 in FIGS. 2A and 2B, the noise pulses 5' and 6' remain large as compared with the demodulated signal waveform 4', in case of low degree of modulation as shown in FIG. 2B, to remarkably degrade the limiting effect, while the noise pulses 5 and 6 can be effectively suppressed as compared with the demodulated signal waveform 4 in case of high degree of modulation as shown in FIG. 2A.
If the inverse biases to the diodes 161 and 162 by the bias sources 17 and 18 are decreased to decrease the limiting range between the limiting levels for the purpose of improving the limiting effect in case of low degree of modulation, the demodulated signal waveform in case of high degree of modulation is also clipped so that the signal waveform is distorted.